The present invention relates to apparatus for chromatography in which the capillary column is heated directly and to a method of controlling the temperature of the capillary column.
It is known in the art that direct heating of the column confers various advantages, among which is a considerable reduction in the quantity of electrical power required to heat the column by means of an element made of electrically conductive material which encircles the same capillary column.
An example of a portable gas chromatography apparatus with direct heating of the column is described in U.S. Pat. No. 5,611,846 by Overton et al. In order to directly heat the column, this document suggests inserting the column into a sheath together with a conductive filament, or inserting the column directly into a tube made of conductive material. The hypothesis of using columns covered with conductive materialxe2x80x94such as, for example, columns in fused silica covered with a thin layer of aluminumxe2x80x94had already been rejected because of frequent breakage of the column or the covering conductor due to the different coefficients of thermal expansion of the materials.
As a temperature sensor, the Overton patent suggests to use a filament made of conductive material inserted into the sheath or the conductive tube in close contact with the column but insulated electrically from the other conductive element (tube or filament) used to heat the column. A control device able to determine the temperature of the conductor as a function of its resistance is employed to regulate the heating of the column. The control device cyclically compares the set temperature with that calculated on the basis of the temperature sensor signal and corrects the power supplied to the heating conductor by varying the voltage applied to the same. Moreover, the possibility of using the same heating conductor as temperature sensor Is also suggested, without however specifying what kind of control device could be suitable for this particular embodiment.
While not specifying which type of control was used In the system of the cited US patent, it was asserted that the system is able to control temperature ramps of slope up to 10xc2x0 C./sec. This parameter confirms that the direct heating of the column can be efficient from the point of view of the rapidity of response of the system, but is opportune to point out that obtaining similar heating speeds with an Inadequate control device could compromise the stability and the precision of the system in Its entirety.
In fact, it has been subsequently found that this type of system has shortcomings-related to both the precision of temperature measurement with respect to conventional chromatography apparatus with oven heating, and to the repeatability of the set temperature profiles for Identical analyses (see xe2x80x9cNovel Column Heater for Fast Capillary Gas Chromatographyxe2x80x9d; Overton et alxe2x80x94-Journal of Chromatographic Sciencexe2x80x94Vol.34 xe2x80x94December 1996, for example). U.S. Pat. No. 5,114,439 by Yost et al describes a chromatography apparatus in which the column is covered with a film of conductive material. This document confirms substantially the effectiveness of the column direct heating technology, but it is opportune to emphasize that the use of columns covered with conductive films involves the disadvantages already identified by Overton referred to previously. As temperature control device, the Yost patent suggests the use of a PID-type industrial controller to control by feedback the electrical supply to the conductor associated with the column.
A system for heating the column by means of a tube conductor which contains the same column is illustrated in U.S. Pat. No. 5,808,178 and in the corresponding international patent application n. WO 97/14957 in the name of Thermedics. The temperature control system alternates cycles of supplying constant high voltage to heat the tube conductor, with measurement cycles of more limited constant voltage to measure the resistance of the tube conductor and, consequently, the temperature of the tube/column system. Moreover, the use of standard PI or PID type standard industrial controllers is suggested as an improvement of a similar system.
However, it should be noted that even PID type standard controllers are inadequate to guarantee the necessary temperature control precision when particularly high heating speeds are applied.
A slightly improved column temperature control system is described in U.S. Pat. No. 5,005,399 by Holtzclaw et al. Also in this case, the heating system provides for the use of a column covered with a conductive film, therefore subject to all the disadvantages already previously cited. However, to control the heating temperature, a control device is suggested in which a pseudo-derivative correction factor is introduced into the feedback control of the temperature or, rather, of the voltage applied to the conductive material which covers the column. One of the main disadvantages of this system lies in the fact that, to maintain the correct operation of the column within the specified limits of error (xc2x11xc2x0 C.), particularly precise calibration of the gain of every component of the control system is required. The calibration operations necessarily demand a certain skill and experience, and they are necessary every time the column is replaced.
The object of the present invention is to propose a chromatography apparatus with direct heating of the column that gives particularly precise control of the temperature of the column.
Another object of the present invention is to propose a chromatography apparatus with direct heating of the column that can guarantee high response speed, maintaining in any case the necessary precision with respect to the set temperature profiles, be they linear or not linear (e.g. exponential and polynomial)
These objects are achieved by the present invention, that relates to a chromatography apparatus, of the type comprising at least one capillary column and means to control the temperature of the column, characterized by comprising at least one element made of electrically conductive material to heat directly the capillary column, and by means for controlling the temperature of the capillary column comprising a control device operating according to a mathematical model having one component of predictive type, or of Feed Forward type, which describes the thermodynamic behavior of the assembly comprising the electrically conducting element and the column at least as a function of the thermal resistance and the thermal capacity of the cited assembly to regulate the supply of electrical energy to the conductive element.
The behavior of the conductive element is therefore simulated by one model component that takes account of the thermodynamic characteristics of the conductor/column assembly to determine what power must be applied to the conductive element to obtain the pre-established temperature at a given time. That in particular allows a fast response to unexpected variations of the temperature profiles set up for the analysis.
Beyond the predictive or Feed Forward type component, the model comprises also one component preferably of corrective or Feed Back type, to correct any errors of temperature that may be Introduced by the predictive component of the model.
Contrary to what happens in the known art, In which the control systems operate in feedback alone trying continuously to reach a series of equilibrium conditions, the system of the present invention operates according to a mathematical model that describes the thermodynamic behavior of the system under the form of a transfer function determined by two very well-defined algorithms.
The advantage of this approach is given by the greater contribution of power being determined a priori by the Feed Forward type model component, while a smaller contribution of power Is determined by the Feed Back type model component on the basis of the temperature error. In this way, the power determined by the Feed Back type model is considerably reduced and the control consequently turns out to be much more stable.
However, It must be taken into account that rapidity and precision are requirements that may be in conflict. In order to obtain rapidity of response it is necessary to supply instantaneously considerable power to the conductive element, while to obtain a certain precision it is opportune to supply limited amounts of power to the heating element over relatively longer times. Therefore, the powers determined on the basis of the two model components can also be xe2x80x9cweightedxe2x80x9d as a function of the requirement that needs to be privileged in any determined application
According to a preferential aspect of the present invention, the temperature control device is able to cyclically update the parameters of the mathematical model and, in particular, at least the values of thermal capacity and thermal resistance.
This gives particularly high precision as far as the regulation of the temperature of the column directly heated by the conductive element is concerned.
In order to detect the temperature of the column, It is preferably used the same element In electrically conductive material destined for the heating of the column. The column and the conductive element are disposed In one covering sheath, In conditions of close mutuat contact for all the length contained Inside the same sheath. This allows effective thermal exchange to be maintained between conductive element and column for all their length and to limit thermal losses.
The covering sheath is preferably made of electrically Insulating material and the column/conductor/sheath assembly can be wrapped In coils without undesired short circuits occurring that might prejudice the correct operation of the system.
In a possible embodiment of the Invention, the element In electrically conductive material is made In the form of a filament. However, It must be understood that the system and the method for controlling the temperature according to the invention are applicable to any type of directly heated column assembly In which is present a conductive element made according to a different form.
Moreover, the control system adopted In the apparatus according to the present invention can also be used in the case in which a further conductive element Is used, separate from the heating element, to detect the temperature of the column.
The covering sheath is made from electrically insulating material and is preferably able to resist high temperatures, such as ceramic fibers or similar, for example.
Alternatively, the sheath can also be made from a thermo-shrinking type of material. The materials currently known having such characteristic, such as those known by the commercial name Teflon(trademark) for example, can not support high temperatures much above 200xc2x0 C., but this does allow the filament and the column to be easily introduced into the inside of the sheath before the assembly thus formed is subjected to heating to allow the contraction of the sheath and to obtain the desired close contact between column and filament.
The invention further relates to a method for controlling the temperature of a capillary column in a chromatography apparatus, characterized by providing for the use of at least one element made of electrically conductive material to heat directly the capillary column, and by the control of the temperature being carried out according to a mathematical model having a component of the predictive type, or Feed Forward type, which describes the thermodynamic behavior of the assembly comprising the electrically conductive element and the column at least as a function of the thermal resistance and the thermal capacity of said assembly to regulate the supply of electrical power to the conductive element.